Process for manufacturing carbazole



Patented Sept. 6, 1949 raoonss eon MANUFACTURING CARBAZOLE Courtney (Jonover, Silver Spring, Md., assignor to Monsanto Chemical Company, St. Louis, Mo, a corporation of Delaware No Drawing. Application August 24, 1948., Serial No. 45,973

Sims. (0!. zen-s) 1 This invention relates to the manufacture of carbazole and especially to processes for manuiactu-ring carbazole from o am-inobiphenyi.

The synthesis of carbazol'e from o-aminoblphenyl by heating the substance in the vapor phase with oxygen in the presence of a catalyst such as an oxide of vanadium or molybdenum was reported by Morgan and Walls (J. Soc. Chem. Ind. 57, 358, 19 38.) The vapor phase reaction produces good conversion at 600 C. However, at temperatures as low as fill-550 C. the conversion is almost negligible. The high temperatures necessary for this conversion, the limitations of apparatus and handling inherent in the use of catalysts in conjunction with high temperature vapor phase operations, the hazard from fires and explosions in vapor phase operations, and other problems make new methods of manufacture of carbazole from o-aminobiphenyl desirable.

Efiorts have been made to obtain carbazole by fusing 'o-aminobiphenyl with a nitro compound of the benzene series at temperatures in the range of 340-350 C. This process results in the formation of large portions of a complex tarry residue and a crude carbazole product grossly contaminated and difficult to purify. Moreover, since the nitro compound employed as an oxidation agent in this process is apparently converted to a complex tarry mixture from which no usable product can be recovered, the cost of this material can only be charged as part of the cost of "raw material, thereby greatly increasing the cost of the finished product.

One of the objects of the present invention is to provide an improved process "for making carbazole by the air-oxidation of o-aminobiphenyl in the liquid phase.

Another object is to provide an improved method of oxidizing oaminobiphenyl in the liquid phase with a minimum formation of tar. Other objects will become apparent from the following description and examples.

This application is a continuation-1mm of my copending application, Serial No. 708,938, filed November 9, 1946, now U. :8. Patent 2599,2211.

According to the present invention, generally stated, o-aminobiphenyl is oxidized to carbazole in good yields by intimately contacting oaminobiphenyl in the liquid phase with air, for

example, by agitating o-aminobiphenyl in the molten state in the presence of a stream of air or other oxygen-containing gas at a temperature in the range of 300-400 C., over a period of at least six hours and generally not over twentyfour hours. As a further embodiment of my invention, the process is desirably conducted in the presence of an inhibitor of tar formation. I have found that either crystalline alumina hydrate or graphite are excellent inhibitors of tar formation in the process of the present invention and are far more efiective than any other substances tested for this purpose.

In the process of the present invention, air may be supplied to the reaction zone under pressure, for example, at a pressure in the range of 10-75 lbs. per sq. in., and desirably in the range of -55 lbs. per sq. in. and thereafter vented from the reaction zone. The air stream may be passed through the reaction mixture, although this is not essential when the reaction mixture is vigorously agitated. On the other hand, it may be desirable to employ the air stream as a means for e fi-ecting at least part of the agitation, for example, by passing the stream through the reaction mixture. The volume of air brought into contact with the reaction mixture by any desired means during the course of the reaction should be in excess of theory for the oxidation of oaminoblphenyl to carbazole and desirably approximately 4 to 6 times the theoretical requirement. .A suitable rate of flow of air for this purpose maybe in the range of 2 to 22 cu. ft. per lbs. of o-aminobiphenyl per minute. This rate of flow maybe varied according to the particular conditions employed in each instance.

The tar inhibitor, for example, the crystalline alumina hydrate or graphite, are desirably employed in amounts of 0.8 part by weight or more per 100 parts by weight of o-amincbiphenyl although smaller or larger amounts may be employe'd, for example, with variations in the temperature, pressure, and agitation. I have found that increasing the reaction temperature above 280 C. results in the formation of less tar, although this is accomplished at the expense of lower conversions of o-aminobiphenyl to carba-- zole. I have also found that the presence of crystalline alumina hydrate or graphite in. amounts hereinbefore set forth results in very marked inhibition of the tar formation, and particularly in the temperature range of 330-375" C.

The carbazcle formed by the process of the present invention may be recovered by any desired procedure, for example, the reaction mixture may be dissolved in hot o-xylene and the solution may be filtered while hot. The carbazole crystallizes from the filtrate and is then filtered off, washed with o-xylene and dried. Other suitable solvents such as hot o-dichlorobenzene may Example I A mixture of 250 grams of o-aminobiphenyl and 4 grams of fine flake graphite, is heated to r a temperature of 375 C. in a jacketed, heated autoclave. The liquid reaction mixture is strongly agitated, for example, by means of a rotary agitator at 960 R. P. M. A stream of air at a pressure of 50 lbs. per sq. in. is admitted to the autoclave at the rate of 2 cu. ft. per minute.

The pressure in the autoclave is maintained at approximately 50 lbs. per sq. in. and the reaction mixture is maintained at 375 C. for 6 hours. Thereafter, the pressure is released and the reaction mixture is dissolved in hOt o-xylene and dried. A 28.2% conversion of o-aminobiphenyl to carbazole is thus obtained representing a yield based on o-aminobiphenyl consumed 93.4%.

Example II A series of runs were made according to the method of Example I in which the pressure in the autoclave was varied. The temperature of the reaction mixture was maintained at 375 C. and the rate of agitation was maintained at 960 R. P. M. The tar inhibitor employed was 4 grams of fine flake graphite, per 250 grams of o-aminobiphenyl. The pressure in the autoclave was varied in successive runs from 10 lbs. per sq. in. to 75 lbs. per sq. in. In each run carbazole was found to have been. produced'in substantial yields. The range of -60 lbs. per sq. in. was found to give very satisfactory results in terms of yield and quality of product.

The process of the present invention, as illustrated in the aforescribed examples is desirably operated for at least about six hours and generally not over twenty-four hours at a reaction temperature in the range of 330-375 C..with strong agitation and with an excess of air over the theoretical requirement in order to obtain a maximum conversion of o-arninobiphenyl to carbazole with a minimum of tar formation and a maximum yield of carbazole based on o-aminobiphenyl consumed. Moreover, for maximum inhibition of tar formation, the process is desirably operated in the presence of fine flake graphite to the extent of approximately 4 parts by Weight of inhibitor to 250 parts by weight of o-aminobiphenyl, or at least 1.6 parts by Weight per 100 parts of o-aminobiphenyl. Variations in I claim:

1. A process of making carbazole comprising intimately contacting o-aminobiphenyl in the liquid phase with an excess of an oxygen-containing gas under pressure in the presence of graphite at a temperature in the range of 300-400 0., fora period of time in the range of 624 hours, and subsequently recovering carbazole from the reaction mixture.

2. A process of making carbazole comprising intimately contacting o-aminobiphenyl in the liquid phase with an excess of air under pressure in the presence of graphite at a temperature in the range of 300-375 C. for a period of time in the range of 6-24 hours, and subsequently recovering carbazole from the reaction mixture.

3. A process of making carbazole comprising intimately contacting o-aminobiphenyl in the liquid phase with an excess of air under presthe conditions under which the process is operated, depending upon the nature and size of the equipment and the other factors involved, are contemplated as being within the scope of the present invention and the invention is not .to be construed as being limited to the specific conditions, agents, or apparatus described in the examples except as defined in the appended claims.

sure in the presence of at least 0.8 part by weight of graphite per parts by weight of o-aminobiphenyl at. a temperature in the range of 330-375 C. for a period of time in the range of 624 hours, passing said air into the reaction mixture at a rate in the range of 2-22 cu. ft. per 100 lbs. of o-aminobiphenyl per minute, and subsequently recovering carbazole from the reaction mixture.

4. A process of making carbazole comprising intimately contacting o-aminobiphenyl in the liquid phase with an excess of air under pressure in the presence of graphite at a temperature in the range of 330-375 0., for a period of time in the range of 6-24 hours, passing said air through said reaction mixture at a rate in the range of 2-22 cu. ft. per 100 lbs. of o-aminobiphenyl per minute, and subsequently recovering carbazole from the reaction mixture.

5. In the manufacture of carbazole by the air-oxidation of o-aminobiphenyl in the liquid phase, the step comprising intimately c0ntacting o-aminobiphenyl in the presence of graphite and in the presence of air at a pressure in the range of 10-75 lbs. per sq. in. and a reaction temperature in the range of 300-375 C. for a period of time in the range of 6-24 hours.

6. In the manufacture of carbazole by the air-oxidation of o-aminobiphenyl in the liquid phase, the step comprising intimately contacting o-aminobiphenyl in the presence of at least 0.8 part by Weight of graphite per 100 parts by weight of o-aminobiphenyl and in the presence of air at apressure in the range of 10-75 lbs. per sq. in. and a reaction temperature in the range of 300-375 C. for a period of time in the range of 6-24 hours.

- COURTNEY CONOVER.

. REFERENCES CITED "The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,351,171 Weinmayr June 13, 1944 2,456,378 Cislak et a1 Dec. 14, 1948 

